LM1279N National Semiconductor, LM1279N Datasheet - Page 7

• Manufacturer Part Number: LM1279N
• Description: IC VIDEO AMP SYS 110MHZ 20-DIP
• Manufacturer: National Semiconductor

Specifications of LM1279N

• Applications: General Purpose
• Number Of Circuits: 3
• -3db Bandwidth: 110MHz
• Current - Supply: 80mA
• Current - Output / Channel: 28mA
• Voltage - Supply, Single/dual (±): 7.5 V ~ 8.5 V
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1279N

Pin
No.
Pin Descriptions
ESD and Arc-Over Protection
16
The ESD cells of the LM1279 are improved over the ESD
cells used in typical video pre-amps. The monitor designer
must still use good PC board layout techniques when de-
signing with the LM1279. The human body model ESD sus-
ceptibility of these parts is rated at 3 kV (Note 4). However,
many monitor manufacturers are now testing their monitors
to the level 4 of the IEC 801-2 specification. This requires the
inputs to the monitor to survive an 8 kV discharge. If the
monitor designer expects to survive such levels he MUST
provide external ESD protection to the video pre-amp inputs!
PC board layout is very important with LM1279 as with other
video pre-amps. The LM1279 provides excellent protection
against ESD and arc-over, but the LM1279 is not a substitute
for good PCB layout.
Figure 6 shows the recommended input protection for a
video pre-amp. The two diodes at the video pre-amp input
and after the 30
against ESD. When this protection is combined with a good
PCB layout, the video pre-amp will easily survive the IEC
801-2 level 4 (8 kV ESD) testing commonly done by monitor
manufacturers. If the protection diodes are moved to the
video input side of the 30 resistor, then the ESD protection
will be less effective. There is also the risk of damaging the
diodes since there is no resistor for current limiting. In such
a design a heavier duty diode, such as the FDH400, should
be used. It is strongly recommended that the protection di-
odes be added as shown in Figure 6 . The 1N4148 diode has
a maximum capacitance of 4 pF. This would have little affect
on the response of the video system due to the low imped-
ance of the input video.
Many monitor designers prefer to use a single zener diode
instead of the recommended two diodes at the video pre-
amp input. The required location of the zener diode is shown
in Figure 6 . It is shown as a dashed line, indicating an alter-
native to the two diode solution. The zener diode does give
the savings of one component, but now the protection is less
effective. To minimize capacitance, the zener diode needs to
have a zener voltage of 24V or higher. This is well above the
V
cated at the video input for protection against a low voltage
surge. The 30 resistor is needed to limit the current of such
a voltage surge, protecting the video pre-amp. Protection
against ESD by using a zener diode is about as effective as
having the two diodes at the video input (same location as
the zener diode). A higher series resistor may be necessary
for protection against the zener voltage, but the higher resis-
tor value will impair the performance of the LM1279; result-
ing in a lower bandwidth and a less stable black level. For
maximum reliability the monitor designer should not consider
the zener diode solution for ESD protection of the LM1279.
The ESD cells of the LM1279 also gives good tolerance
against arc-over. Once again the monitor designer must be
careful in his PCB layout for good arc-over protection. In the
video chain only the outputs of the CRT driver are directly
exposed to the voltages that may occur during arc-over. A
good PCB layout is the best protection for the video pre-amp
CC
voltage of the LM1279. The zener diode must be lo-
V
CC2
Name
Pin
series resistor offers the best protection
(Continued)
Schematic
7
against arc-over. The pre-amp vulnerability is mainly through
the ground traces on the PCB. For proper operation all
ground connections associated with the video pre-amp, in-
cluding the grounds to the bypass capacitors, must have
short returns to the ground pins. A significant ground plane
should be used to connect all the pre-amp grounds. Figure
16 , the demo board layout, is an excellent example on an ef-
fective ground plane, yet using only a single sided PCB lay-
out. Here is a check list to make sure a PC board layout has
good grounding:
If any one of the above suggestions are not followed, then
the LM1279 may become vulnerable to arc-over. Improper
grounding is by far the most common cause of a video pre-
amp failure during arc-over.
Functional Description
Figure 1 on the front page shows the block diagram of the
LM1279 along with the pinout of the IC. Each channel re-
ceives a video input signal at its input amplifier (-A1). The
output of the input amplifier goes to the contrast attenuator
stage. For easy interfacing to 5V DACs all controls inputs, in-
cluding the contrast control, use a 0V to 4V range. The con-
trast control has no attenuation with an input of 4V, and has
full attenuation (over −40 dB) with a 0V input. All three chan-
nels will accurately track the contrast control setting at pin
10. Each channel will have the same amount of attenuation
for a given input voltage typically to within
nels will track because the contrast control is the first stage
of attenuation and the internal control voltage generated
from the input voltage is common to all three channels.
The output of the contrast attenuator goes to the drive at-
tenuator. This stage has a 12 dB control range. This stage is
used for color balance, so the adjustment range has been
limited to 12 dB for a more accurate color balance. Each
• All associated grounds with the video pre-amp are con-
• CRT driver ground is connected to the video pre-amp
• CRT and arc protection grounds are connected directly to
FIGURE 6. Recommended Video Input ESD Protection
nected together through a large ground plane.
ground at one point.
chassis, or the main ground. There is no arc-over current
flow from these grounds through the pre-amp or CRT
driver grounds.
Power supply pin for the output stage.
There are no internal connections to V
Description
±
0.3 dB. All chan-
DS100091-12
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CC1
.